1. Field of the Invention
The present invention relates in general to optical communication. More particularly, the invention is directed to an optical communication system and an optical communication method. An optical signal is directly intensity-modulated by an analog signal and is transmitted through an optical communication means wherein the modulation distortion of signals being communicated is reduced by application of feedforward control.
2. Description of the Prior Art
With the development of optical fiber communication networks optical cable television (hereinafter CATV), wherein audio/video information is transmitted through an optical fiber instead of a coaxial cable, has been proposed. Using optical fibers permits transmission of an increased amount of information, enabling a large number of additional transmission channels as well as extended transmission distances.
Known transmission systems for optical CATV include a wideband frequency modulation method (hereinafter FM) and a vestigial sideband amplitude modulation system (hereinafter VSB/AM). An FM system is advantageous in that signal-to-noise ratio is large and intermodulation distortion is small, but requires an AM/FM converter for converting an electrical amplitude-modulated signal (hereinafter AM signal) into a frequency-modulated signal (hereinafter FM signal) when injecting the optical signal into the optical fiber as well as an FM/AM converter for converting the optical FM signal received over the optical fiber into the electrical AM signal. This necessarily gives rise to the overall high cost of the communication system. Conversely, VSB/AM system requires no converters as discussed above, thereby allowing both an inexpensive and simple construction of the communication system. With the VSB/AM system, the current injected into a light emitting element, for example, a semiconductor laser is directly modulated to produce the optical signal. However, the VSB/AM system suffers from a problem that the light output contains high order distortions that result in a poor CN (carrire-to-noise) ratio (equivalent to SNR, signal-to-noise ratio) since the injected current versus light output characteristic of the light emitting element such as the semiconductor laser is non-linear.
The amount of light output does not increase in proportion to the amount of current injected into the semiconductor but will contain the modulation distortion components proportional to the square of the injected current. Therefore, when transmitting information using a carrier frequency f.sub.1, if the frequency components of 2f.sub.1, 3f.sub.2, f.sub.1+f.sub.2, 2f.sub.1 -f.sub.2, and so on are developed and these frequencies exist in the proximity of other frequencies, the modulation distortion components become a noise source for the other carrier frequencies to degrade the CN ratio thereof. Thus the following relation applies to transmitting picture information. EQU 10.multidot.log.sub.10 (amplitude of carrier/amplitude of modulation distortion).gtoreq.58 (dB) (1)
According to this relation, it is necessary to restrict the number of channels for transmission. Two ways of preventing deterioration of CN ratio were known; one in which the semiconductor laser as a light source is biased by a constant current and the light developed is externally modulated using an amplitude modulator that exhibits a linear modulation characteristic, the other in which so-called light feedforward (hereinafter FF) discussed in the paper "ANALOG TRANSMISSION OF TV-CHANNELS ON OPTICAL FIBER WITH NON-LINEARITIES CORRECTION BY REGULATED FEEDFORWARD" by J.P. FRANKART et al. REV. H.F. ELECTRON TELECOMMUNICATION, VOL. 12, Nov. 9, 1984. In the feedforward system, the electrical signal to be transmitted is applied to the semiconductor laser. Part of the main optical signal through the semiconductor laser is compared with the original electrical signal to obtain a corresponding correction signal. Then, the modulation distortion produced in the semiconductor laser is suppressed by adding the correction signal thus obtained to the main optical signal to perform so-called feedforward control thereof.
It is theoretically possible to greatly reduce the modulation distortion if the aforementioned feedforward system is constructed as described. However, deterioration with age and environmental changes cause alterations in the light emitting characteristics of the light emitting means such as the semiconductor laser, especially the slope efficiency (coefficients of the first order terms of a function of the input current and the light output). In such cases, with the aforementioned light feedforward method, operating parameters that have been initially set to an ideal condition may deviate from that condition due to the changes in the characteristics, thereby increasing the modulation distortion. Conventionally, no practical measures have been taken against changes in the parameters of the light communication system due to such environmental changes, as it has not been possible to sufficiently suppress the modulation distortion.
In the feedforward system, since signal propagation time depends on transmission paths of the respective signals and signal processing mechanisms inserted therein, the difference in the signal propagation time should be taken into consideration. One problem which has been encountered in connection with the aforementioned light feedforward system where the correction signal is added to the main signal in order to eliminate the distortion modulation, is that signal delay due to such difference in signal propagation time causes insufficient suppression of the modulation distortion components or even may increase the distortion.
Therefore, in the light feedforward system, an electrical delay circuit is provided at a stage immediately before the signals are added together to thereby adjust the propagation time relative to each other. However, no investigation has been carried out for the signal delay, especially the range of delay and the accuracy of delay. For this reason, the modulation distortion could not be suppressed adequately.
Moreover, the signal delay is adjusted at the receiving end, requiring a large construction of the receiving station. The miniaturization of the receiving station has been difficult.